Liquid crystal cells for active matrix TV and computer monitors are made of two glass plates sandwiching a layer of liquid crystal material between them. The glass plates are made conductive with a thin conductive film on the inside faces of the plates so that a source of power may be connected to them for changing the orientation of the liquid crystal molecules. As the need for larger and more sophisticated cells that allow separate addressing of different areas of the liquid crystal cell has progressed, as for active matrix TV where up to 1,000,000 or more different areas or pixels need to be separately addressed, the use of thin film transistors for this application has come into widespread use. Thin film transistors comprise a patterned metal gate over which is deposited a gate dielectric layer and a conductive layer, such as amorphous silicon. Subsequently applied layers, as of doped amorphous silicon, etch stopper silicon nitride, silicon oxide, metal contact layers and the like, are also required to be deposited over the amorphous silicon thin film. Many of these films are deposited by CVD in order to obtain high quality films.
In the semiconductor industry, as substrates such as silicon wafers have become larger, permitting a greater number of devices to be formed on a wafer, single substrate processing has largely replaced batch type processing of several wafers at a time. Single substrate processing allows greater control of the process, permits smaller vacuum chambers to be used and, if a problem arises during processing, only a single wafer, rather than a whole batch of wafers, is damaged or lost.
To improve the productivity of a single substrate vacuum processing system, including evacuating and re-pressurizing the processing chamber after each substrate has been processed, vacuum equipment has been designed that includes more than one processing chamber and a transfer chamber, so that multiple step processes can be performed in different chambers on a single substrate without removing the substrate from a vacuum environment. Such a system also has the concomitant advantage of a cleaner system. For example, Maydan et al have disclosed such a system in U.S. Pat. No. 4,951,601, which comprises a central transfer chamber surrounded by and connected to various processing chambers. A robot in the transfer chamber transfers the substrates from one chamber to another. The elimination of the need for evacuating the chambers prior to each processing step by the addition of a vacuum load lock also increases the throughput of the equipment.
Glass is a brittle dielectric material that requires slow heating and cooling, e.g., about 5 minutes or more, to avoid cracking or stressing of large glass plates over the temperature range of from room temperature up to about 300.degree.-450.degree. C., typically used for vacuum processing. Since the actual thin film deposition requires only seconds, without special provisions being made, a lot of idle time in the vacuum system would occur while the substrates are being individually heated and cooled. This waiting time would be very costly in terms of lost reactor time, and thus deposition of films in single substrate chambers would not be economical.
The deposition of multiple layer films on single glass substrates in a single vacuum system has been disclosed for example by Gallego, U.S. Pat. No. 4,592,306. The vacuum system disclosed by Gallego includes four or more deposition chambers connected to a central transfer chamber, a loading chamber and an exit chamber. Substrates are loaded into the system in the loading chamber which is evacuated, and the substrate is transferred by means of a robot in the central transfer chamber successively to two or more deposition chambers where various layers are deposited. The exit chamber can double as a metal deposition chamber. The sequential thin films are deposited onto the substrates which are loaded in the deposition chambers one at a time. The system was designed for sequentially depositing intrinsic and doped amorphous silicon layers for the manufacture of solar cells. Deposition is by glow discharge of silane and suitable dopant gases.
This system, while effective to deposit sequential layers on large glass substrates in a single vacuum system without breaking vacuum, is uneconomic because of the long period of time required to process each substrate and it does not provide heating and cooling of substrate materials. Gallego addresses part of this problem by providing two chambers for the deposition of intrinsic amorphous silicon, which layer is thicker and thus requires a longer deposition time than the thinner, doped amorphous silicon layers. However, Gallego did not address ways of reducing the overall deposition time or how to bring the temperature of the substrates to the reaction temperature, (270.degree. C.) nor the time required to cool the substrates back to ambient temperatures prior to removing the substrate from the vacuum system.
Thin film transistors cannot be made using glow discharge techniques since amorphous silicon films made by glow discharge of silane have a high hydrogen content, which makes for unstable films and thus unstable transistor devices. Since CVD processes require higher temperatures than glow discharge deposition, on the order of 350.degree.-450.degree. C., and the glass substrates useful for active matrix TV monitors typically are quite large, e.g., 350.times.450.times.1 mm, it requires several minutes to heat the substrates up to processing temperatures and cool them down to ambient temperatures again after film processing is completed. This heating and cooling delay would be very costly in terms of lost reactor times. Thus deposition in several chambers would not result in efficient operation without waiting, unless the long heating and cooling times are addressed.
Thus a vacuum system having improved throughput that can process large glass substrates in a series of single substrate processing chambers and that solves the heating and cooling delay time would be highly desirable.